Method of tin-antimony alloy plating



.METHOD OFTlN-ANTIMGNY ALLOY PLATING Frederick A. Lowenheirn, Plainfield, and Herbert B. Forman, Hillside, N. J., assignors to Metal & Thermit Corporation, New York, N. Y., a corporation of New ersey No Drawing. Application March 22, 1954 Serial N 6. 417,955

2 Claims. (Cl. 204-43) This invention relates to a method for producing tin plate resistant to transformation of the tin. Tin transformation, sometimes referred to as tin pest or tin disease, is a phenomenon according to which tin articles or tin plated articles show blistery outbreaks or flakes on their surface; the outbreaks may spread until the whole surface becomes brittle and crumbles to a gray powder which lacks the metallic luster of ordinary tin. Tin transformation occurs in cold climates, the transformation reaching a maximum rate at about minus 48 C. for dry tin.

According to the invention, tin deposits may be produced which are resistant to transformation by electrodepositing the tin from an alkaline stannate electroplating bath incorporating about 0.1 to about 10 g./l. of dissolved sodium or potassium antimonate. The electrodeposition results in a codeposit of a tin-antimony alloy, with the antimony comprising only about 0.2 to about 1% of the alloy. The alloy so produced has been found to be resistant to tin transformation.

The preferred alkaline stannate electroplating bath is the sodium stannate bath containing about 0.25 to about 0.5 mol per liter of sodium stannate and about 0.2 to about 0.6 mol per liter of free sodium hydroxide. With this bath sodium pyroantimonate is the preferred antimonate salt since its concentration in the bath is selfregulating; an amount of sodium pyroantimonate is added suflicient to saturate the bath solution and to provide therein an undissolved excess of sodium pyroantimonate. Amounts of dissolved sodium pyroantimonate as low as 0.1 g./l. may be employed, but preferably the amount is the saturation concentration. Tin may be electrodeposited from the sodium stannate bath at a temperature in the range of about 60 C. to substantially the boiling point of the bath solution and at a current density in the range of about to 30 amperes per square foot (a. s. f.). A- preferred temperature is in the range of 70 to 90 C. and a. preferred current density is 12 to 20 a. s. f.

The plating bath may also be made up from potassium stannate, instead of sodium stannate, in which case its composition may comprise about 0.25 to about 3.0 mols per liter of potassium stannate and about 0.15 to about 3.5 mols per liter of free potassium hydroxide; the temperature of the potassium bath may range from about 50 C.

to substantially the boiling point of the bath solution and the current density from about 30 to about 1000 a. s. f.; and the antimonate salt is used in small quantities rang"- ing from about 0.1 to about g./l. of dissolved salt. Potassium pyroantimonate is preferred for this bath. The antimonate salt must be replaced as antimony is plated out.

Other suitable antimonate salts are sodium meta-antimonate and potassium meta-antimonate. Also, the antimonate salt may be supplied by adding to the sodium stannate or potassium stanate plating solution a pentavalent antimony compound which will react with the solution to form a sodium or potassium antimonate in situ. Thus, antimony pentachloride added to a sodium stannate plat- 7, 2,825,583 .P ten edM 4,1 58

ing solution forms sodium antimonate together with sodium chloride; an antimonate of an alkaline earth metal like calcium, barium, strontium, or magnesium forms sodiurnantimonate and a stannateof the alkaline earth metal; ammonium pyroantimonate forms sodium antimonate and'ammonia; and reactions corresponding to the tin maybeplated, including tin itself.

"The'following' examples may illustrate the invention.

Example 1 Sodium dihydrogen pyroantimonate, usually referred to as sodium pyroantimonate, was prepared by first dissolving 50 grams of potassium dihydrogen pyroantimonate in one liter of boiling water. Then a sodium carbonate solution was prepared by dissolving grams of sodium carbonate in 400 cc. of water. The sodium carbonate solution was added to the pyroantimonate solution and a precipitate of sodium pyroantimonate formed which was filtered and washed. About one-fourth of this wet precipitate was then added to one liter of a conventional sodium stannate electroplating bath containing 105 g./l. of sodium stannate (0.39 mol) and 9.1 g./l. of sodium hydroxide (0.23 mol). The quantity of sodium pyroantimonate added to the plating bath was considerably in excess of that required to saturate the bath, the excess being allowed to settle out of the bath solution and to remain on the floor of the bath. Using a pair of tin anodes and a 2 x 3" tin panel (identified as No. 1) as cathode, this bath was electrolyzed at a temperature of 70 C. and a current density of 12 a. s. f. for about 15 minutes to produce a tin-antimony codeposit on the panel comprising about 0.2% of antimony and the balance tin. Using the same bath, three other tin panels, identified as Nos. 2, 3 and 4, were electroplated, each over a period of about 15 minutes. In No. 2, the current density Was 24 a. s. f. and the temperature 80 C.; the codeposit contained 0.4% of antimony. In No. 3, the current density was 30 a. s. f., the temperature was C., and the codeposit contained 0.5% of antimony. In No. 4, the current density was 24 a. s. f., the temperature was 88 C., and the codeposit contained 0.2% of antimony. In all cases the appearance of the codeposit was satisfactory, being indistinguishable from that of pure tin.

Example 2 A sodium stannate bath the same as that described in Example 1 was prepared and two 4" x 8" brass panels (Nos. 5 and 6) were electroplated in it for 25 minutes at 12.4 a. s. f. and 8690 C. Four other 4" x 8" brass panels (Nos. 7, 8, 9, and 10) were electroplated in the bath each for 15 minutes at 24.8 a. s. f. and 88-9l C. Panel No. 11, identical to Nos. 5 to 10, was plated in a sodium stannate bath identical to that described, except that it contained no sodium pyroantimonate, and under the same plating conditions as were used for panel Nos. 7 to 10. The codeposit on each panel was about 0.3 mil thick. All seven panels were tested for tin transformation. A simple procedure for this test is to attach to each deposit a seed crystal of alpha tin and to place each panel in a cold cabinet maintained at about minus 40 C. In panel No. 11 the transformation was efiected in about 24 hours, whereas in panel Nos. 5 to 10 no transformation had occurred at the end of six months, at which time the tests were discontinued.

Besides tin anodes, insoluble anodes like steel, graphite, etc. are suitable for use in the process.

In the light of the foregoing description, the following is claimed:

trodepositing said plate on an article cathode from a bath solution containing about 0.25 to about 0.5 mol .per liter of sodium stannate, about 0.2 to about 0.6 mol per liter of free} sodium hydroxide, and an amount of sodium pyroantimonate sutficient to saturate the solution and to provideQan undissolved excess of sodium pyroantimonate, electrodepositing said tin at a temperature in the range of about '60 C. to substantially the boiling point of-the solution and at'a current density'in the'rangeof about 5 tor30 amperes per square foot.' r 1 a '2. Method of producing tin-antimony alloy plate cont aining about to .oi'f antimony, said plate being retaining about 0.2 to 1% of antimony, said plate being resistant to transformation of the tin, which comprises elecsistant to transformation of the tin, which comprises electrodepositing said plate on an article cathode from a bath solution containing about 0.25 to about 3.0 mols per liter of potassium stannate, about 0.15 to about 3.5 mols per liter of free potassium hydroxide, and about 0.1 to about 10'g./1. of dissolved potassium pyroantimonate, electrodepositing said tin at a temperature in the range of about 50C. to substantially the boiling'point of the solution and at a current density in therange'of about 30 to about 1000 amperes per square foot;

7 References Cited in the file of this patent Monk et all: Transactions Faraday Society, vo1. 31 1935 pp. 14 54, p 

1. METHOD OF PRODUCING TIN-ANTIMONY ALLOY PLATE CONTAINING ABOUT 0.2 TO 1% OF ANTIMONY, SAID PLATE BEING RESISTANT TO TRANSFORMATION OF THE TIN, WHICH COMPRISES ELECTRODEPOSITING SAID PLATE ON AN ARTICLE CATHODE FROM A BATH SOLUTION CONTAINING ABOUT 0.25 TO ABOUT 0.5 MOL PER LITER OF SODIUM STANNATE, ABOUT 0.2 TO ABOUT 0.6 MOL PER LITER OF FREE SODIUM HYDROXIDE, AND AN AMOUNT OF SODIUM PYROANTIMONATE SUFFICIENT TO SATURATE THE SOLUTION AND TO PROVIDE AN UNDISSOLVED EXCESS OF SODIUM PYROANTIMONATE, ELECTRODEPOSITING SAID TIN AT A TEMPERATURE IN THE RANGE OF ABOUT 60*C. TO SUBSTANTIALLY THE BOILING POINT OF THE SOLUTION AND AT A CURRENT DENSITY IN THE RANGE OF ABOUT 5 TO 30 AMPERES PER SQUARE FOOT. 